As far as catalysts for copolymerization of ethylene with alpha-olefins are concerned, three properties of the catalysts are of a major importance:
1) molecular weight distributions of the resins produced with the catalysts, PA0 2) the response of the resin molecular weight to hydrogen, PA0 3) ability of the catalysts to effectively copolymerize ethylene and alpha-olefins.
One of the measures of the molecular weight distribution of linear low density polyethylene (LLDPE) resins is the melt flow ratio (MFR), which is the ratio of the high-load melt flow index (I.sub.21) to the melt index (I.sub.2) for a given resin: MFR=I.sub.21 /I.sub.2. The MFR value is believed to be an indication of the molecular weight distribution of a polymer: the higher the MFR value, the broader the molecular weight distribution.
Molecular weight of ethylene copolymers can be controlled in a known manner, e.g., by using hydrogen. With the catalyst compositions produced according to the present invention, molecular weight can be suitably controlled with hydrogen when the polymerization is carried out at temperatures from about 30.degree. to about 105.degree. C. This control may be evidenced by a measurable positive change in the I.sub.2 and I.sub.21 values of the polymers produced. A relatively high sensitivity of the resin molecular weight to the amount of hydrogen present during the polymerization process is an important feature of the catalyst compositions of this invention.
Still another important property of catalyst compositions for ethylene/alpha-olefin copolymerization is the ability thereof to effectively copolymerize ethylene with higher alpha-olefins, e.g., C.sub.3 -C.sub.10 alpha-olefins, to produce resins having low densities. This property of the catalyst composition is referred to as "higher alpha-olefin incorporation property" and is usually measured by determining the amount of a higher alpha-olefin (e.g., 1-butene, 1-hexene or 1-octene) required in a polymerization process to produce a copolymer of ethylene and the higher alpha-olefin having a given copolymer composition and a given density. The lesser is the amount of a higher alpha-olefin required to produce the resin of a given density, the higher are the production rates and, therefore, the lower is the cost of producing such a copolymer. Effective higher alpha-olefin incorporation is especially important in the gas-phase fluidized bed process, because relatively high concentrations of higher alpha-olefins in the fluidzed bed reactor may cause poor particle fluidization.
It is an object of the present invention to provide high-activity catalyst compositions for copolymerization of ethylene and alpha-olefins yielding products with bimodal molecular weight distributions.
It is an additional object of the present invention to provide a catalytic process for copolymerizing ethylene and alpha-olefins which yields LLDPE with a bimodal molecular weight distribution at high productivity.
It is also an object of the present invention to provide high activity catalyst compositions for the copolymerization of ethylene and alpha-olefins which exhibit a relatively high melt flow index response to hydrogen.
The beneficial effect of DMAC as a cocatalyst component has been examined. In copolymerization reactions, catalyst compositions containing DMAC exhibit the properties of good alpha-olefin incorporation, and, more significantly, produce resins with broad or bimodal molecular weight distributions. As shown in the FIGURE the products of DMAC-cocatalyzed ethylene copolymerizations contain a high molecular weight component; this high molecular weight component can account for the increased MFR values attributable to the products compared to products produced with trialkylaluminum cocatalysts. The products of DMAC-cocatalyzed ethylene copolymerizations exhibit processability advantages and superior mechanical properties compared to resins cocatalyzed by triethylaluminum (TEAL) or trimethylaluminum (TMA). Specifically, the DMAC-cocatalyzed products exhibit excellent gloss and low haze characteristics as well as excellent dart impact resistance.
However, DMAC as a cocatalyst component exhibits less activity than trialkylaluminum compounds. Moreover, the catalyst compositions containing DMAC alone as a cocatalyst exhibit decreased hydrogen response. Moreover, the DMAC cocatalyst under certain polymerization conditions exhibits a significant propensity for production of alpha-olefin oligomers. The oligomers foul gas-phase fluidized bed polymerization reactors and cause reactor shutdowns.